So-called directly converting radiation converter materials are known for the detection of gamma or X-ray radiation. Individual quantum absorption events can be detected using such directly converting radiation converter materials. Radiation detectors based on such radiation converter materials are usually also referred to as counting detectors. The directly converting radiation converter materials are generally semiconductor materials in which gamma or X-ray radiation is converted into electrical charge carriers in a single conversion step.
A special situation is present when such a radiation converter material is used for a radiation converter or for a radiation detector for a human-medical X-ray tomography apparatus, for example of a computed tomography apparatus. Here quantum absorption events have to be detected quantitatively or in a counting fashion at comparatively high quantum flux rates of e.g. more than 108 X-ray quanta/mm2*s.
Limits of the quantitative detection arise on account of different boundary conditions. Radiation converter materials have defect sites, for example in the form of vacancies or interstitial atoms, in a manner governed by production. These are responsible for polarization effects that lead to a reduction of the charge carrier lifetime/mobility product (μτ product) and thus to an increase in the average residence duration with at the same time a reduction in the lifetime of the charge carriers in the semiconductor material. Polarization effects thus reduce the separation efficiency of the liberated charge carriers and lead to a widening of the detected electrical signal. As a result there is a risk, in particular, of signals from quanta that arrive in close temporal succession being superposed in such a way that it is no longer possible to separate the events. However, liberated charge carriers can also recombine with oppositely charged defect sites present. Depending on the charge carrier lifetime, these charge carriers are then lost for conversion into an electrical signal.
Radiation converter materials are known in which the material is doped with a dopant with the aim of minimizing the polarization effects. In this case, the doping atoms introduced are intended to passivate or compensate for the defect sites present in the crystal as completely as possible. However, this optimization approach generally leads simultaneously to an undesirable decrease in the ohmic resistivity of the radiation converter material. As a result of the high applied voltage for separating the liberated charge carriers by means of the electric field thus generated, a comparatively high dark current or leakage current is thus associated with this. This leads to a reduction of the signal-to-noise ratio. The spectral sensitivity of the radiation detector and, consequently, the detectability of low-energy X-ray quanta are greatly reduced as a result.
Taking this as a departure point, the intention is to provide a directly converting radiation converter material which is matched to a typical quantum flow in human-medical X-ray examinations with regard to rate and spectral distribution. Furthermore, the intention is to provide the corresponding radiation converter and a radiation detector. Moreover, the intention is to specify a method for producing such a radiation converter material.